The present invention relates generally to industrial scales for measuring large loads through the use of electromechanical load cells and more particularly the invention relates to mounting apparatus for positioning such load cells for use in such scales to improve the accuracy and longevity of such load cells.
A load cell, as used in the scales to which the present invention pertains, is critically calibrated at the manufacturer's laboratory. The load cell must be mounted at specified precise points and it must be loaded at a specified point. The particular type load cell to be used with the present invention is a double-end shear beam load cell. This type of load cell may be mounted in one of two ways, to wit: each end of the load cell may be affixed to a support structure and the load supplied to the load cell on the top thereof at the center line; or the load cell may be mounted to the support structure in the middle and accept the load at each end of the cell. Mounting the cell in the second manner requires a more expensive load cell. The load cell should not be permitted to move, swing or twist in the mounting inasmuch as any variation from the exact points of loading and mounting will cause weighing errors and/or damage to the cell. The cell should also be under no pressure in the mounting inasmuch as a binding condition will result if the cell is fastened down too tightly to the stand; yet, if it is too loose, the load cell will rock or twist and either condition causes errors and/or damage.
Conventionally when the load cell accepts the load in the center, the ends of the load cell are bolted or pinned down to a rigid stand. A weigh bridge on the scale is supported on a pedestal or on a girder chair which in turn is supported by a type of pin or link which rests directly on the load cell. The weigh bridge receives shock in the course of loading the scale and moves when the scale is loaded. This shock and movement is transmitted through the pedestal directly to the load cell, which is ideally stationary. When the weigh bridge moves, the pedestal moves and consequently the point of loading on the load cell also moves; therefore inaccuracies in the scale measurement result. To compensate for the movement, prior art has used a plurality of check stays to attempt to stabilize the weigh bridge. Such compensation creates additional problems inasmuch as the steel weigh bridge may be damaged if the check stays hold it too rigidly or the weigh bridge may bind against the stay which causes weighing inaccuracies.
When the load cell is configured to accept the load from the weigh bridge on each end thereof, the cell is mounted to a rigid stand at its center. Links are suspended directly from the load cell at each end. These links are connected to a common lower weight bearing pin which extends beyond each end of the load cell. The pedestal supporting the weigh bridge rests on this lower pin and again the movement of the pedestal is transferred to the lower pin. The lower pin exerts downward and lateral force on the links which swing directly on the load cell. The links are swinging and also twisting thus creating excessive wearing conditions between the links and the load cell and varying the point of loading on the load cell thus resulting in inaccuracies. As noted in the alternative method of mounting, the shock from the weigh bridge is transferred to the load cell, through the load cell into the stand and then rebounds from the stand into the load cell. Since the standard mountings apply the load directly from the scale weigh bridge pedestal to the load cell, the shock is substantial. The damage which occurs to the load cell due to the shock of loading can be extensive. In addition the rigid fastening of the load cell to the steel in the stand below the load cell results in a second shock when the shock rebounds from the rigid stand.